1. Field of the Invention
This invention relates generally to circuits employing transformers as a circuit element and more particularly to transformer circuits arranged to improve the signal-to-noise ratio of an output signal.
2. Description of the Prior Art
A transformer is a device comprising a winding with a tap or taps, or two or more coupled windings, with or without a magnetic core, for introducing mutual coupling between electric circuits. In the prior art, transformers are commonly used to provide an impedance transformation, voltage and current transformations, and to isolate one section of a circuit from undesired influences produced in other sections of the circuit. In communication networks, it is desirable to utilize iron or ferrite cores having magnetic properties for reducing transformer size and stray flux. However, most magnetic core transformers produce a frequency dependent or nonlinear magnetizing current conducted through a frequency dependent or nonlinear magnetizing inductance causing a distortion of the output signal and the generation of signals at undesired frequencies. Transformer circuits such as that described in the U.S. Pat. No. 3,881,149 utilize an active compensation circuit to provide a negative inductance for cancelling the transformer magnetizing inductance to linearize the response of iron core transformers as a function of frequency, as a function of current, or as a function of both frequency and current. However, spurious signals at undesired frequencies from an external source may still be conducted by the compensated transformer circuit to a load.
It is therefore desirable to arrange a transformer circuit including a transformer having a magnetic core and an active circuit to compensate for transformer core losses and the nonlinearities in the response of the transformer circuit due to the utilization of transformer magnetic cores exhibiting nonlinear current characteristics. In addition, the transformer circuit should attenuate spurious signals at undesired frequencies.